1. Field of the Invention
The present invention relates to a waterproof connector.
2. Description of the Related Art
A gas sensor (hereinafter, also referred to as a sensor) for detecting a specific component of exhaust gas usually employs a waterproof structure for isolating an interior space (hereinafter, also referred to as an internal space) of the gas sensor casing from the exterior thereof to prevent foreign matter, such as water, from entering into the casing. When an engine having such a sensor attached to an exhaust pipe of the engine is running, a front end of the sensor is exposed to high-temperature exhaust gas. Thus, due to thermal conduction, the internal space of the sensor assumes a high temperature. When the engine is shut off, the internal space is cooled. In order to prevent expansion and contraction (variation in pressure) of air contained in the internal space (hereinafter, also referred to as internal air) caused by such thermal variation, the sensor, for example, may allow for exchange of internal air through its rear end portion. Specifically, the sensor may employ a filter which is permeable to air, but is impermeable to liquid.
On the other hand, in order to meet a demand for reduction in size and enhancement of waterproofness, a certain sensor (hereinafter, also referred to as a completely waterproof sensor) has a closed internal space without employing a filter for exchange of internal air. In sensors including such a completely waterproof sensor, a waterproof connector is usually attached to end portions (terminals) of electric wires extending outwardly therefrom. The waterproof connector is fitted, for connection, to a counterpart waterproof connector attached to external terminals of electric wires extending outwardly from an engine control unit (ECU).
In the case where electric wires of the completely waterproof sensor are connected to corresponding electric wires by use of waterproof connectors, the presence of an opening (defect) in an insulating coating layer of an electric wire connected to a counterpart waterproof connector, through which opening an internal core wire(s) is exposed to outside air, causes the following phenomenon in association with variation in air pressure of the internal space of the sensor during use. When the air pressure of the internal space becomes high as a result of exposure to heat, air contained in the internal space flows through an electric wire (i.e., through small clearances between a large number of the core wires constituting the electric wire) covered with the insulating coating layer, and flows out of the electric wire from the opening. In contrast, when the internal space assumes a negative pressure (vacuum) as a result of cooling, the outside air is drawn in from the opening. Accordingly, when water is splashed over the opening or when the opening is immersed in water, water is drawn in from the opening and flows through the electric wire toward the internal space of the sensor. As a result, when the completely waterproof sensor is repeatedly used, metal terminals or the like contained in the waterproof connectors (hereinafter, also referred to as connectors) and the sensor may corrode.
Measures to solve the above problem include use of a waterproof connector having a filter which is impermeable to liquid, but is permeable to air for allowing air flow between the interior and the exterior of the connector (as disclosed, for example, in Patent Documents 1 and 2). In the waterproof connector disclosed in Patent Document 1, a sealing rubber plug for preventing entry of foreign matter such as water is fitted into an idle cavity (hole) of a housing into which a metal terminal is not inserted; a through-hole extending between the interior and the exterior of the hole is formed in the rubber plug; and a filter which is permeable to air, but is impermeable to liquid, is provided in the through-hole.
In the connector disclosed in Patent Document 2, a sleeve-type filter (seal member) which can restrict passage of liquid and is permeable to air is arranged between an electric wire (lead wire) and a wall of an electric-wire insertion hole of a casing (hereinafter, referred to as housing) through which the electric wire is inserted, such that the outer circumferential surface of the filter is in close contact with the wall of the hole.
In the case where electric wires of the above-mentioned completely waterproof sensor are connected to corresponding electric wires by use of the waterproof connector disclosed in Patent Documents 1 or 2, provision of the air-permeable filter (or seal member) in the connector solves the above-noted problem due to variation in pressure within the sensor in a cooling-heating cycle, thereby preventing entry of foreign matter such as water into the connector or the sensor through the electric wire. That is, even when an opening (defect) is present in an electric wire connected to a counterpart waterproof connector, and the opening is immersed in water, water is not drawn in toward the sensor.
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2003-331979
[Patent Document 2] Japanese Patent Application Laid-Open (kokai) No. 4-249079
3. Problems to be Solved by the Invention:
However, in the waterproof connector disclosed in Patent Document 1, the filter is provided in the sealing rubber plug which is inserted into the idle cavity (unused hole) into which no metal terminal is inserted. Accordingly, the waterproof connector employs a number of holes greater than the number of electric wires to be inserted into the holes. That is, in the waterproof connector of Patent Document 1, since the sealing rubber plug is inserted into a hole into which no metal terminal is inserted, an idle hole must also be provided. In other words, a connector of greater size must be used which is disadvantageous.
In the connector disclosed in Patent Document 2, even when a plurality of electric wires are inserted into a respective plurality of holes, the filters must be inserted between the wall surface of each hole and the outer circumferential surface of each electric wire. Thus, the filter attachment structure employed by the waterproof connector of Patent Document 2 suffers from poor waterproof reliability and high cost.